A typical convex lens has a uniform radius of curvature and is characterized by a single focal length. However, there are optical applications that require or benefit from the use of miniature focusing elements having more than one focal length. For example, the depth of focus for an optical system can be increased by using a lens having more than one focal length. Increasing the depth of focus in an optical system reduces the loss of image sharpness that may result from misalignment. For laser optical systems, multiple focal lengths can be used to provide control over the power density distribution along the beam axis.
For purposes of the present application, a “miniature optical element” refers to an optical element having a maximum dimension of less than five millimeters. Fabricating miniature optical elements with multiple focus points is very challenging. Currently, diffractive lenses can be fabricated that provide multiple focal lengths; however, diffractive elements are typically lossy and inefficient. Since as much as fifty percent of incident light can be diffracted in non-useful directions, miniature diffractive lenses are not well suited to applications in which transmission losses are a major concern, such as in optical telecommunications systems.